Published byStanford Medicine

Here’s something that caught my attention this morning (likely because I’m the mom of two girls): A new study provides a possible reason behind reports that parents with firstborn daughters are more likely to divorce than those with firstborn sons. According to researchers from Duke and University of Wisconsin-Madison, it could be due to girls being “hardier than boys, even in the womb.”

Throughout the life course, girls and women are generally hardier than boys and men. At every age from birth to age 100, boys and men die in greater proportions than girls and women. Epidemiological evidence also suggests that the female survival advantage actually begins in utero. These more robust female embryos may be better able to withstand stresses to pregnancy, the new paper argues, including stresses caused by relationship conflict.

Based on an analysis of longitudinal data from a nationally representative sample of U.S. residents from 1979 to 2010, Hamoudi and Nobles say a couple’s level of relationship conflict predicts their likelihood of subsequent divorce.

Strikingly, the authors also found that a couple’s level of relationship conflict at a given time also predicted the sex of children born to that couple at later points in time. Women who reported higher levels of marital conflict were more likely in subsequent years to give birth to girls, rather than boys.

“Girls may well be surviving stressful pregnancies that boys can’t survive,” Hamoudi said. “Thus girls are more likely than boys to be born into marriages that were already strained.”